Those skilled in the art know that white light can be made from an equal mixture of the primary colors red (R), green (G), and blue (B). If one primary color is removed from white light, a complimentary color (also known as a subtractive primary or secondary color) is obtained. The subtractive primary colors are yellow (Y, i.e., white minus blue), magenta (M, i.e., white minus green), and cyan (C, i.e., white minus red). When lights of three different primary colors are seen together, a new color is produced by an additive process. But when pigments, such as printing inks, are added to white paper, the resulting colors are produced by a subtractive process.
The basic principle of color reproduction through the media of modern color printing of pictures lies in the use of only three primary colors to create the illusion of seeing, in the reproduction, the many more colors that the eye sees in the original. Pure colors are said to be saturated. Colors may be de-saturated by addition of white ink or black ink (K). The saturation of a color is an indication of how far that color is from gray. Virtually all commercial color printing processes use, in addition to the primary colors, black ink and, therefore, such processes are referred to as four-color processes.
The first step in any color reproduction system is called "color separation". More sophisticated and contemporary methods use photo electronic scanners. Photoelectronic scanning of a color image typically produces a separation of that image into red, green, and blue components which are output as RGB signals. In order to reproduce such an image in printed form, the RGB signal must be converted into a CMY signal which can be read by a suitable color printing apparatus. Theoretically, there should be a one-to-one correspondence between the primary colors of red, green, and blue to the printing inks of cyan, magenta, and yellow. This is given by the following relationship: ##EQU1## In practice, however, there exist certain deficiencies which prevent printer's inks (CMY) from producing accurate neutral gray tones. To correct for this, black (K) ink is added to assist in the reproduction of neutral tones.
Currently, most color separation systems (including classical photographic and current electrical, mechanical and computer based methods) produce the black layer of a separation by a process known as "Under Color Removal" (UCR). UCR generally defines the amount of black ink used at a particular location as a function of one of the three (CMY) ink densities. A typical function used to identify which CMY will be used in the UCR computation is: EQU f(C,M,Y)=min.(C,M,Y) (1)
That is, UCR generally determines how much black ink to use at a particular location by first determining which of the three ink (CMY) densities is the lowest. The black density is then taken to be a fixed percentage of that lowest density.
To correct for this additional ink, some amount of ink based upon the black density value is subtracted from each of the three (CMY) color densities and the amount of colored ink used at the location of interest is adjusted accordingly. This is why the process is termed "under color removal."
Unfortunately, the UCR method perturbs the color ratios of the cyan, magenta, and yellow inks such that that ratio is different than that of the corresponding RGB colors. This is because the UCR method requires removal of an equal amount of ink from each of the three (CMY) inks. Therefore, a slightly different hue (i.e., the dominant wavelength) is obtained than what is wanted and additional color correction is required. The UCR method also applies more black ink than is necessary in the middle color density ranges; this has a tendency to dull those colors. Moreover, there is deficiency in the UCR method, in that, the black ink does not produce as deep a black as one would expect or desire. This is adjusted, under the UCR method, if the percentage of black is less than 100 percent, by further application of equal portions of the three-color inks in addition to black.
Others (See e.g. U.S. Pat. Nos. 4.700,399 and 4,803,496) have attempted to improve upon the UCR method. However, to the applicant's knowledge, no one has successfully developed a method for converting the RGB color image to four color printed image which preserves the color ratios and maintains the desired hue. The present invention provides for an apparatus and method for converting RGB images to four color printed images which overcomes the deficiencies of the prior art.
Thus, it is an objective of the present invention to convert RGB images to four color printed images while maintaining the color ratios of the original RGB image;
It is further objective of the present invention to use black ink only when necessary and to reduce the total amount of ink used in the printing process;
It is still a further objective of the present invention to reduce the number of color correction steps needed to convert an RGB image to a four color printed image, thereby reducing the complexity of the apparatus required and reducing the cost of such four color printing.